Fix preloading polymorphic association when through association has already loaded

If through association has already loaded, `source_type` is ignored to
loaded through records. The loaded records should be filtered by
`source_type` in that case.

Fixes #30904.

2 files changed, 27 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/associations/preloader/through_association.rb b/activerecord/lib/active_record/associations/preloader/through_association.rb
index fa32cc5553..5bd49b041a 100644
--- a/activerecord/lib/active_record/associations/preloader/through_association.rb
+++ b/activerecord/lib/active_record/associations/preloader/through_association.rb
@@ -13,18 +13,30 @@ module ActiveRecord
         end
 
         def associated_records_by_owner(preloader)
+          already_loaded = owners.first.association(through_reflection.name).loaded?
           through_scope = through_scope()
 
-          preloader.preload(owners,
-                            through_reflection.name,
-                            through_scope)
+          unless already_loaded
+            preloader.preload(owners, through_reflection.name, through_scope)
+          end
 
           through_records = owners.map do |owner|
             center = owner.association(through_reflection.name).target
             [owner, Array(center)]
           end
 
-          reset_association(owners, through_reflection.name, through_scope)
+          if already_loaded
+            if source_type = reflection.options[:source_type]
+              through_records.map! do |owner, center|
+                center = center.select do |record|
+                  record[reflection.foreign_type] == source_type
+                end
+                [owner, center]
+              end
+            end
+          else
+            reset_association(owners, through_reflection.name, through_scope)
+          end
 
           middle_records = through_records.flat_map(&:last)
 
diff --git a/activerecord/test/cases/associations/nested_through_associations_test.rb b/activerecord/test/cases/associations/nested_through_associations_test.rb
index 4cf5a9ffc4..11ee45917f 100644
--- a/activerecord/test/cases/associations/nested_through_associations_test.rb
+++ b/activerecord/test/cases/associations/nested_through_associations_test.rb
@@ -579,6 +579,17 @@ class NestedThroughAssociationsTest < ActiveRecord::TestCase
     assert !c.post_taggings.empty?
   end
 
+  def test_polymorphic_has_many_through_when_through_association_has_already_loaded
+    cake_designer = CakeDesigner.create!(chef: Chef.new)
+    drink_designer = DrinkDesigner.create!(chef: Chef.new)
+    department = Department.create!(chefs: [cake_designer.chef, drink_designer.chef])
+    Hotel.create!(departments: [department])
+    hotel = Hotel.includes(:chefs, :cake_designers, :drink_designers).take
+
+    assert_equal [cake_designer], hotel.cake_designers
+    assert_equal [drink_designer], hotel.drink_designers
+  end
+
   def test_polymorphic_has_many_through_joined_different_table_twice
     cake_designer = CakeDesigner.create!(chef: Chef.new)
     drink_designer = DrinkDesigner.create!(chef: Chef.new)